This invention relates generally to the production of viscous hydrocarbons from subterranean hydrocarbon-containing formations. Deposits of highly viscous crude petroleum represent a major future resource in the United States in California and Utah, where estimated remaining in-place reserves of viscous or heavy oil are approximately 200 million barrels. Overwhelmingly, the largest deposits in the world are located in Alberta Province Canada, where the in-place reserves approach 1,000 billion barrels from depths of about 2,000 feet to surface outcroppings and at viscosities of up to 1 million c.p. at reservoir temperature. Until recently, the only method of commercially recovering such reserves was through surface mining at the outcrop locations. It has been estimated that more than 90% of the total reserves are not recoverable through surface mining operations. Various attempts at alternative, in-situ methods, have been made, all of which have used a form of thermal steam injection. Most pilot projects have established some form of communication within the formation between the injection well and the production well. Controlled communication between the injector and producer wells is critical to the overall success of the recovery process because in the absence of control, injected steam will tend to override the oil-bearing formation in an effort to reach the lower pressure area in the vicinity of the production well. The result of steam override or breakthrough in the formation is the inability to heat the bulk of the oil within the formation, thereby leaving it in place. Well-to-well communication has been established in some instances by inducing a pancake fracture. However, often problems arise from the healing of the fracture, both from formation forces and the cooling of mobilized oil as it flows through a fracture towards the producer. At shallower depths, hydraulic fracturing is not viable due to lack of sufficient overburden. Even in the case where some amount of controlled communication is established, the production response is often unacceptably slow.
U.S. Pat. No. 4,037,658 to Anderson, specifically incorporated herein by reference, teaches a method of assisting the recovery of viscous petroleum, such as from tar sands, by utilizing a controlled flow of hot fluid in a flow path within the formation but out of direct contact with the viscous petroleum; thus a solid-wall, hollow tubular member in the formation is used for conducting hot fluid to reduce the viscosity of the petroleum to develop a potential passage in the formation outside the tubular member into which a fluid is injected to promote movement of the petroleum to a production position.
The method and apparatus disclosed by the Anderson patent and related applications is effective in establishing and maintaining communication within the producing formation, and has been termed the Heated Annulus Steam Drive, or "HASDrive", method. In the practice of HASDrive, a hole is formed through the petroleum-containing formation and a solid wall hollow tubular member is inserted into the hole to provide a continuous, uninterrupted flow path through the formation. A hot fluid is flowed through the interior of the tubular member out of contact with the formation to heat viscous petroleum in the formation outside the tubular member thereby reducing the viscosity of at least a portion of the petroleum adjacent the outside of the tubular member, creating a potential passage for fluid flow through the formation adjacent the outside of the tubular member. A drive fluid is then injected into the formation through the passage to promote movement of the petroleum for recovery from the formation.
U.S. Pat. No. 4,565,245 to Mims describes a well completion for a generally horizontal well in a heavy oil or tar sand formation. The apparatus disclosed by Mims includes a well liner, a single string of tubing, and an inflatable packer which forms an impervious barrier and is located in the annulus between the single string of tubing and the well liner. A thermal drive fluid is injected down the annulus and into the formation near the packer. Produced fluids enter the well liner behind the inflatable packer and are conducted up the single string of tubing to the wellhead. The method contemplated by the Mims patent requires the hot stimulating fluid be flowed into the well annular zone formed between the single string of tubing and the casing. However, such concentric injection of thermal fluid, where the thermal fluid is steam, could ultimately be unsatisfactory due to scale build up in the tubing or the annulus. This scale comprises a deposition of solids such as sodium carbonate and sodium chloride, normally carried in the liquid phase of the steam as dissolved solids, which are deposited as a result of heat exchange between the fluid in the tubing and the fluid in the annulus. Parallel tubing strings, as disclosed in U.S. Pat. No. 4,595,057 to Deming, is a configuration in which at least two tubing strings are placed parallel in the well bore casing. The use of parallel tubing has been found to be superior in minimizing the scaling and heat loss suffered by prior injection methods during thermal well operations.